Ever since Joseph Priestly discovered how to "impregnate water with fixed air" in the 18th century, carbonated beverages have been ubiquitous. Sodas have been used to cure diseases, fight alcoholism, and spread American culture around the globe. On this week's Please Explain, we’ll find out what soda is, what’s in it, and when it became so popular. We’re joined by Dr. Kelly Brownell, the co-founder and director of the Yale Rudd Center for Food Policy and Obesity, and Darcy O'Neil, beverage master and author of the book Fix the Pumps, a history of soda from the 18th century to the present.